Process for determining an operating parameter of an internal combustion engine as a function of three control parameters of this motor. More particularly, such a procedure permits determining the angle of the ignition advance (xcex1) as a function of the speed of rotation of the engine (N), of the motor load (intake pressure P) and of the air/fuel ratio (xcex lambda).
In indirectly injected internal combustion engines, it is known to determine the angle xcex1 of ignition advance as a function of the speed of rotation of the engine N and of the quantity of air P admitted. Thus, so that this type of engine can operate correctly, it is necessary that the richness of the mixture R be substantially constant and equal to 1. Because of this, the parameter xcex (lambda), which is the inverse of this richness 1/R, does not vary or varies only little, and it is not or almost not necessary to take account of it to determine the angle of ignition advance xcex1. Thus, this angle of ignition advance xcex1 is determined on a test bed for an engine type and its value is then applied to all the engines of the same type with the help of maps connecting the angle of ignition advance xcex1, the engine speed N and the quantity of air introduced into the cylinder P.
Such maps are now made and although they require a large number of operating points (N, P) their construction and use do not give rise to great difficulty.
It is known from EP 302 735 to compute two correction maps (so-called high degree and low degree maps) so as to determine the instant of ignition in each cylinder of the motor. The air/fuel ratio is modified in accordance with at least one of the maps. These maps thus permit switching between two different octane indices. But they do not permit specifying a correction factor which would be a function of this octane index. Thus, in the scope of this document, only two octane indices are necessary.
It is also known from EP 892 161 to use two separate maps to determine the instant of ignition. The first corresponds to operation of the engine according to a homogeneous combustion mode and the second corresponds to operation with a rich mixture (which is to say during switching between the stratiphied mode and the homogeneous mode). The passage from one map to the other takes place by linear interpolation of a parameter representative of the engine operation. Unfortunately, when the engine operating parameter does not vary in a linear manner, it is thus impossible to use these two maps, because otherwise the values computed for this parameter will differ too much from real values.
In the case of internal combustion engines using direct fuel injection in each of the cylinders and/or in those using a lean mixture, such maps become useless. Thus, such engines have a large range of variations of xcex. Because of this, the angle of ignition advance no longer depends solely on the motor speed (N) and its load (P), but it also depends and largely on xcex. However, memorization of tables in a four-dimensional space (xcex1, N, P, xcex) is very difficult to carry out in a simple manner. Standard microprocessors used on central control units of the operation of the engine do not permit memorizing and processing such four-dimensional maps. These maps thus require large memory resources, their management requires too much computation and depends on a particular symbolism for their representation which is not available in conventional microprocessors.
The object of the present invention is thus to represent in a fashion which will be the simplest possible, the most precise and the least costly in computation time, a four-dimensional space. In this space, three of the axes define control parameters of the motor (for example N, P, xcex) and permit determining a fourth variable (for example xcex) which is a parameter of operation of this motor.
It is already known, to this end, to determine an additive or proportional correction factor as a function of xcex, established from a single table connecting the advance correction a and the parameter xcex at an operating point (N, P). However, this type of correction does not permit covering wide ranges of variation of xcex, particularly it is not possible with such a correction to define a correction factor valid for all the points (N, P) of operation. Because of this, the ignition advance angle becomes imprecise if xcex is far from the basic lambda and/or if it is a matter of an operation point (N, P) very different from that for which the correction factor has been defined.
It is also known to establish two maps giving the ignition advance angle xcex1 as a function of the motor speed N and of the motor load P, a first map corresponding to a minimum xcex and a second map corresponding to a maximum xcex. As a function of the real value of xcex, one shifts from one to the other of these maps. However, such an operating mode has no finesse as to computation of the ignition advance angle, it is not possible to take account of the present real value of xcex.
The object of the present invention is to determine the ignition advance angle xcex1 accurately as a function of N, P and xcex. More precisely, the present invention seeks to establish in a manner that is simple and less costly as to computation time and memory, a four-dimensional table.
To this end, the present invention relates to a process for determining an operating parameter (xcex) of an internal combustion engine as a function of three control parameters (N, P, xcex) of this engine, characterized in that it consists in:
establishing a first map of the operating parameter (xcex1) as a function of two of the control parameters (N, P), the third control parameter (xcex) being fixed at a first value,
establishing a second map of the operating parameter (xcex) as a function of the two same control parameters (N, P), the third control parameter (xcex) being fixed at a second value,
establishing a relationship between the operating parameter (xcex1) and the third control parameter (xcex) over all the range of variation of this parameter at at least one specific point of operation (N, P),
using this relationship to determine the operating parameter (xcex1) as a function of the three control parameters (N, P, xcex) for all points of operation of the motor.
It is to be noted that the process according to the invention can be applied to the determination of other parameters than those given by way of example, particularly his process also permits computing the Exhaust Gas Recirculation (EGR) level for exhaust gases, in which four parameters are to be followed, but also the advance correction as a function of the EGR level or of the VVT (Variable Valve Timing: modification of the intake diagram) factor and of any other operational parameter depending on more than two control parameters.